Iain Grant (who I think Steve may have met) usually played a set of tremolo diatonics on a six-way spindle. You can't get your hand behind the moothie that way so I guess it must be much like a rack? Iain was a superb melodic player, and while I never saw him use a rack, I know he did once - he entered a trad music duet competition, between himself on moothie and himself on electronic bagpipes.